PROJECT SUMMARY/ABSTRACT Occurring on every continent except Antarctica, West Nile virus (WNV) is the most widely distributed zoonotic arthropod-borne virus (arbovirus) resulting in an estimated 780,000 human illnesses and 1,549 deaths in the U.S. since 1999. WNV is maintained in a Culex mosquito and bird enzootic transmission cycle which is the same cycle for a suite of other parasites and pathogens. As a result, the Culex mosquitoes responsible for enzootic transmission and bridge transmission to humans are frequently exposed to a variety of other parasites and pathogens. Prior studies have demonstrated that co- infections of multiple pathogens in insects can result in direct and immune-mediated interactions. In mosquitoes, these interactions could have important consequences on the ability of an arbovirus to replicate and infect the salivary glands (i.e. vector competence). Additionally, co-infections could influence survival or the time it takes for the virus to reach the salivary glands (i.e. extrinsic incubation period) which are important parameters in the model that describes the number of new hosts exposed to a pathogen by a specified population of mosquitoes per infected host per day (i.e. vectorial capacity). The goal of this proposal is to 1) evaluate the consequences of pathogen interactions on WNV vector competence and vectorial capacity and 2) apply epidemiological models to assess the ability of co-circulating parasites to impact the reproductive number (R0) of WNV in nature. Using laboratory infection experiments in Culex mosquitoes, we will test the hypothesis that co-infections with Plasmodium or an insect-specific flavivirus will significantly increase or decrease WNV vectorial capacity. At the completion of these studies, it is my expectation to have identified specific co- circulating pathogens that have important consequences for WNV transmission in nature. Although these common parasites and pathogens may not directly cause human disease, I anticipate highlighting the potential for these pathogens to affect human health indirectly, by driving dynamics of WNV. During this award, the Principal Investigator (PI) will receive structured training in virology, vector competence, and mathematical modeling and will then apply those skills to the research outlined here. This award will provide a foundation to help the PI achieve his career goals of understanding mechanisms of arboviral transmission to develop improved interventions aimed at mitigating human and animal disease.